job-exec: fix potential hang after exec kill error

Problem: When the job-exec module handles an error from the wait-all
composite future created from exec_kill(), it makes an assumption that
all child futures are also fulfilled, and calls flux_future_get()
ion each individual future in bulk_exec_kill_log_error() to print a
specific error. However, if bulk kill operation times out, then one
or more of the child futures may not be fulfilled, and the module
could block forever.

Check that each child future is ready in bulk_exec_kill_log_error()
before calling flux_future_get() to avoid the hang.

Fixes flux-framework#5523

src/modules/job-exec/bulk-exec.c

@@ -541,7 +541,8 @@ void bulk_exec_kill_log_error (flux_future_t *f, flux_jobid_t id)
     const char *name = flux_future_first_child (f);
     while (name) {
         flux_future_t *cf = flux_future_get_child (f, name);
-        if (flux_future_get (cf, NULL) < 0) {
+        if (flux_future_is_ready (cf)
+            && flux_future_get (cf, NULL) < 0) {
             uint32_t rank = flux_rpc_get_nodeid (cf);
             flux_log_error (h,
                             "%s: exec_kill: %s (rank %lu)",